1. Field of the Invention
The present invention generally relates to methods and apparatus for measuring the chemical mechanical polishing (CMP) process endpoint on a semiconductor wafer during the planarization process, and more particularly, to methods and apparatus for generating an X-ray beam and directing the beam onto the surface of the workpiece to be measured, and analyzing the reflection of the X-ray beam to obtain real-time CMP endpoint (i.e., process endpoint) information.
2. Description of the Related Art
The production of semiconductor devices begins with the creation of high quality semiconductor wafers. Because of the high precision required in the production of these semiconductor devices, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. CMP is often used to remove material from the surface of the wafer to provide a relatively flat surface.
Such polishing is well known in the art and generally includes placing one side of the wafer in contact against a flat polishing surface, and moving the wafer and the polishing surface relative to each other. A slurry, including abrasive particles and/or chemicals that react with the material on the wafer surface to dissolve the material, may also be placed in contact with the wafer surface to assist in removing a portion of the material. During the polishing or planarization process, the wafer is typically pressed against the polishing pad while the pad rotates. In addition, to improve the polishing effectiveness, the wafer may also rotate and oscillate back and forth over the surface of the polishing pad.
A well prepared polishing pad facilitates the uniform, high precision planarization of wafers. This is particularly important when polishing down the material layer on a semiconductor wafer during the manufacture of semiconductor devices.
Presently known methods for measuring the thickness of a material layer on a semiconductor wafer involve measuring the total thickness of an applied material layer, determining the desired thickness of the material layer after planarization, calculating the pressure to be applied during the polishing or planarization process, and further calculating the approximate time required to remove a predetermined amount of material layer for a given pressure and slurry combination. Once the desired removal rate (often expressed in nanometers per minute) is ascertained, a statistical inference is employed to determine the approximate amount of time necessary to remove a desired amount of material. After the wafers have undergone planarization for an amount of time calculated to remove a desired thickness of the material layer, the wafers or workpieces are removed from the machine and the actual thickness of the material layer is measured off-line, for example, through the use of laser interferometric techniques. If it is determined that the material layer is still too thick after initial planarization, the workpieces must be reinstalled onto the CMP machine for further material layer removal. If, on the other hand, an excessive amount of material layer has been removed, it may be necessary to scrap the wafers, resulting in substantial unnecessary costs.
In general, visible light is not able to sense changes in thickness of the material layer, unless the layer is less than approximately 600 Angstroms thick. Ultraviolet light and x-ray beams can penetrate these material layers when they are greater than 600 Angstroms thick. Specifically, x-ray techniques, as used in well known x-ray fluorescence (XRF) measurement systems, can effectively and non-destructively measure the thickness and composition of the material layer once the wafer is removed from the CMP machine. However, this technique has not been utilized in-situ in conjunction with a CMP machine during the planarization or polishing of the wafer for real-time measurement of the thickness of the material layer on the surface of a wafer.
An apparatus is thus needed which accurately measures the material layer thickness (and particularly the CMP process endpoint) during the CMP process (in-situ) to overcome the shortcomings of the prior art.
The present invention provides an improved method and apparatus for measuring the chemical mechanical polishing (CNIP) process endpoint on a workpiece during the planarization process. More particularly, the present invention provides a device to measure the thickness of a material layer on the surface of the workpiece by generating an x-ray beam, directing the beam onto the surface of the workpiece, and analyzing the resultant fluorescent beam to obtain real-time CMP endpoint information including the thickness of the material layer.
In accordance with an exemplary embodiment of the present invention, an x-ray probe assembly is mounted onto a CMP machine. The x-ray probe assembly comprises an x-ray emitter, an x-ray detector, and suitable x-ray optics as required including collimators and lenses. The x-ray emitter is configured to generate and direct an incident x-ray beam onto a location on the surface of the workpiece as the workpiece is being polished. The incident x-ray beam is scattered from and absorbed into the surface of the workpiece and a resultant fluorescent beam is produced. The x-ray detector is configured to receive the resultant fluorescent beam, and the resultant fluorescent beam is then processed by a processor to determine the thickness and composition of the material layer or layers at the incident location on the surface of the workpiece.